This invention relates to a method and circuit for driving an ink jet printer which comprises an electro-mechanical transducer which operates in response to electrical pulses to effect ink jet printing on a medium.
There has been proposed an ink jet printer, as illustrated in FIG. 1, which comprises a pressure chamber 1 containing ink, an ink nozzle 3 connected to the pressure chamber 1, through which the ink is ejected, and a piezoelectric crystal 5 which is actuated by electrical driving pulses to eject the ink from the ink nozzle onto a printing medium 10. When the ink is ejected from the ink nozzle 3, the ejected ink which is separated from the remaining ink in the pressure chamber 1 forms ink droplets. The pressure chamber 1 is connected to an ink supply source (not shown) by way of an ink supply passage 2.
The circular disc shaped piezoelectric crystal 5, together with a metal plate 6 which is rigidly connected to the crystal 5, forms an electro-mechanical transducer 4. The transducer 4 is connected to an electrical driving circuit (not shown). The metal plate 6 is electrically grounded and a positive voltage is applied to the crystal 5. When positive voltage driving pulses are supplied to the crystal 5, the crystal 5 is mechanically contracted, so that the metal plate 6 is bent toward the pressure chamber 1 to press the ink in the pressure chamber 1. As a result, the ink in the pressure chamber 1 is ejected from the ink nozzle 3, in the form of ink droplets. Then, when the voltage which has been applied to the crystal 5 is removed, the crystal 5 is returned to its original position, the metal plate 6 returns to its original position, and the pressure in the pressure chamber 1 decreases. Thereafter, the pressure chamber is refilled with ink through the passage 2 by the capillary force at the ink nozzle 3.
In the prior art, a driving pulse having a wave form as illustrated in FIGS. 2a, 2b, or 2c is applied to the transducer 4 to operate the same. That is, one pulse is applied to the transducer for one ejection of the ink from the nozzle. However, in the prior art, a satellite droplet or satellite droplets 20' tends or tend to occur behind the main ink droplet 20, as illustrated in FIG. 3. When the ink is ejected from the nozzle 3, the front end of the ink moving out of the nozzle 3 is first connected to the ink in the pressure chamber 1 by a long thin tail 22, and is then separated from the ink in the pressure chamber 1, so that the ink droplet 20 and the satellite droplets 20' are produced. The satellite droplets 20' have a bad effect on the appearance of the printed surface of the medium 10 and, accordingly, it is preferable that no such droplets be produced.
Furthermore, the long thin tail 22 which is located in the nozzle 3 often shifts from the center line l--l of the nozzle 3, by the distance e, due to "surface tension", as illustrated in FIG. 4. In addition, the meniscus of the ink often becomes distorted, when the nozzle 3 has a rough surface on the inner face thereof or when the nozzle 3 has a shifted center. The shifted tail 22 or the distorted meniscus causes the ink droplet to be shifted from the center line l--l of the nozzle 3, as illustrated in FIG. 4(b), so that the shifted ink droplet will be applied at an incorrect printing position of the medium 10 (FIG. 1).
Furthermore, after the ink droplet is separated from the remaining ink in the pressure chamber 1 at the tail 22, air may penetrate the ink in the pressure chamber 1, when the plate 6 is returned to its original position, so that air bubbles are produced in the ink. The air bubbles decrease the pressure which is applied to the ink in the pressure chamber 1, since a part of the pressure is absorbed by the air bubbles which can be compressed.
In order to eliminate the above mentioned drawbacks, a so called Stemme type print head for a ink jet printer has been proposed (See Japanese Patent Laid Open No. 48-9622). That print head comprises an inner ink chamber (i.e. pressure chamber) and an outer ink chamber. These ink chambers are interconnected by a first nozzle, and the outer ink chamber is also connected to a second nozzle for ejecting the ink. In this print head, since the tensile force for displacing the ink in the second nozzle toward the first nozzle is relatively small, the ink droplet is separated from the ink at the outlet portion of the second nozzle that is located adjacent to the ejection opening of the second nozzle, which results in the prevention of the occurrence of the long thin tail mentioned above. However, the Stemme type print head is complex and it is very difficult to align the first and second nozzles.